This invention relates to a phase shifter circuit for controlling the power components and input power factor of a cycloconverter or other converter operating on infinite bus. More particularly, the invention relates to a fast response phase shifter circuit for generating cycloconverter reference signals for independently controlling the real power and input power factor or reactive power.
In a power system in which a number of phase-controlled cycloconverters operate with low frequency outputs in parallel, such as paralleled variable speed constant frequency (VSCF) converters wherein each individual capacity is small compared to the total system capacity, then a cycloconverter can be considered to be operating essentially on an infinite bus. The situation is analogous to a synchronous machine operating on an infinite bus, and real and reactive power in the cycloconverter have been controlled by adjusting the amplitude and phase of the induced voltage. Frequency and voltage control have been employed in a manner similar to that used in conventional synchronous machine systems. The frequency control circuit used a phase locked loop system incorporating R-C phase shifting networks and discriminators in the feedback loop. The voltage control circuit was identical to that of a conventional cycloconverter except that the feedback signal was derived from a discriminator which received polarized circulating currents. These circuits are complicated, have slow response characteristics, and additionally are sensitive to frequency drift and have serious mutual interactions.
The prior art control schemes are not suitable for cycloconverters with faster response characteristics, as for example the cascaded high frequency link cycloconverter system described in U.S. Pat. No. 3,742,336 to B. D. Bedford and allowed application Ser. No. 419,490 now U.S. Pat. No. 3,882,369, by W. McMurray, both assigned to the assignee of this invention. In the usual mode of operation supplying power to the load, the input cycloconverter operates in regenerative fashion, however reverse power flow is possible and the control should accommodate power flow in either direction. For such a converter system with or without the output cycloconverter in which the high frequency link parallel resonant circuit is operated at a variable frequency, usually in the kilohertz range, the control circuit should be distortion free, have an almost instantaneous response characteristic, and be insensitive to supply voltage drift. Additionally, the real and reactive power or input power factor should selectively be controllable independently and conveniently.